Low-Field AC Transport Coefficients

Abstract

The drift velocity attained by the carriers in the presence of an external field is determined by the free time available to the electrons between collisions during which they are accelerated by the field. The effect of a time-dependent field may therefore be expected to be different from that of a DC field if the magnitude of the field changes appreciably in the time between collisions. The mean collision time for electrons in semiconductors ranges between 10⁻⁹ and 10⁻¹² seconds and hence a difference between AC and DC transport coefficients may be expected to start from frequencies in the microwave range. Semiconductor devices applicable for generating and amplifying microwave signals have been developed in the last few years [9.1]. An interest in the electron transport properties in semiconductors at microwave frequencies has grown with the emergence of these devices. However, even before the development of microwave semiconductor devices, transport coefficients at microwave frequencies had been extensively studied. These studies were undertaken mainly for two reasons. First, the microwave transport coefficients may be measured without attaching contact leads to the samples [9.2]. For samples in which the production of an ohmic contact is technologically difficult, microwave methods therefore provide a good alternative to DC methods for the measurement of the parameters. The sensitivity of the measuring instruments may also be much improved with microwave signals by using the high-Q value of microwave cavities. Hence, the conductivity of small-sized samples or the Hall mobility of low-mobility samples may also be conveniently measured by the microwave methods [9.3].